In pixel based printing systems such as thermal transfer printing which utilise a carrier or web which carries print medium such as ink (known in the art of thermal printing as "foil"). Conventionally the maximum speed at which information can be printed on a substrate has been determined by the speed at which the print head has been able to print i.e. the operational speed. In thermal printing apparatus the operational speed is determined by, amongst other things, the time it takes to energise and de-energise the printing elements of the head.
Two main alternative thermal printing systems exist. Firstly, printing systems are known in which the substrate, which may for example be a flexible packaging web, and the ink carrier are intermittently held stationary. Whilst they are stationary the print head is traversed relative to the substrate and carrier whilst some or all of a plurality of the printing elements of the print head are energised to transfer pixels of ink from the carrier onto the substrate. In such arrangements the print head cannot be traversed across the carrier and substrate faster than the operational speed of the print head.
Secondly, printing systems are known in which the print head is held stationary whilst a substrate, which may again be a flexible packaging web, and the ink carrier are continuously moved past the stationary print head whilst some or all of the printing elements of the print head are energised. In such a system the substrate and carrier cannot be continuously moved past a stationary print head faster than the maximum operational speed of the print head.
Hence in the first existing system, the speed which information can be printed is limited by the operational speed of the print head, and in the second system, the speed at which the substrate can be moved past the print head is limited by the operational speed of the print head.
There exist in significant numbers, continuous motion packaging machines whose material web linear speeds are much higher than any attainable operational print speeds of even the most advanced high speed thermal transfer print heads. Thus to date thermal transfer printers have only been able to handle such applications by means of additional web control equipment which effectively brings the continuous motion web to rest momentarily in order for the thermal transfer printer to print in the same way as it would in an intermittent motion machine. Clearly this "interference" with the continuous web is unwelcomed and adds considerably to the installed cost of the printing system.
Furthermore end users often utilise both intermittent and continuous motion flexible packaging machines in their production facilities and ideally one would employ a single kind of printing apparatus for both types of machines, in order to minimise the cost of spares and maintenance and maximise operator efficiency.